Definitions of Transmission Summary Parameters

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Transmission Summary parameters found in UV-Lens Data entries

As graphs can be difficult to interpret and are too big to be included in texts, digested numeric values are needed for use elsewhere. They also provide a quick, useful summary of a particular lens's UV transmittance capability.

 

Range.

The approximate wavelength range within which the lens transmits any UV.

See remarks below about the left-hand endpoint, λ_UVzero(nm), of the range.

 

T_VISmax(%)

The highest transmission found in the visible wavelength range (400 - 700 nm).

This value, when compared to the next two values, gives a general idea of the quantitative losses in the UV passband. Generally, we like to find UV-capable lenses which require only +3 stops more exposure length than the corresponding visible exposure of the same subject.

 

T_365nm(%)

Transmission at 365nm.

This value is a good indicator for how much the exposure time is affected for a UV shot when using standard UV-pass filtration around 360 nm and/or UV-LED illumination peaking at 365 nm.

(This value might be obtained with simpler tools than a spectrometer by comparison to a well known lens.)

 

T_400nm(%)

Transmission at 400nm.

For most non-specialized lenses this is the peak UV transmittance point. This value is a good indicator for how much the exposure time is affected for a UV shot for UV-marginal lenses or filtration peaking close to 400nm.

(This value might be obtained with simpler tools than a spectrometer by comparison to a well known lens.)

 

λ_{UV HMvis}(nm)

The wavelength where the UV transmission below 400nm is 50% of the visible max, T_VISmax(%).

This value is similar to the FWHM (full-width half-maximum) for bandpass filters.

Observe this is not at 50% total transmission, making it more meaningful as it relates to the passband level.

 

λ_{UV HM400}(nm)

The wavelength where the transmission below 400nm is 50% of the (max) UV transmission, T_400nm(%).

This value gives us a feel for how quickly the lens's UV transmittance drops.

 

λ_{UV Zero}(nm)

The wavelength where the transmission asymptotically goes to zero%, passing 1%.

A lens having a deep UV "reach" may not necessarily be useful near that zero point. So we must look for those lenses having a steep shoulder just past their zero cut-in points. For normal UV-photography the 0% cut-in alone gives an inflated view of the usable wavelength range as the light from longer wavelengths will normally dominate the image when using broadband UV illumination.